The invention describes a method for making a braze joint, and more particularly, to a method of making and controlling the formation of a braze utilizing alternating current resistive heating.
Brazing is a process for joining materials, either ceramic or metallic, which relies on the melting, flow and solidification of a filler metal to form a hermetic seal, a structural bond, or both. Brazing generally requires capillary flow of liquid filler metal through a gap, joint clearance, or other discontinuity between similar or dissimilar base materials. As the filler metal advances through the discontinuity, the surface energies at the liquid-solid-gas interface define the time-dependent contact angle at this triple point phase convergence and also along the free surface contour (that is, the fillet). The properties and quality of the resulting braze joint depend on the braze flow characteristics (such as braze viscosity and surface energies) and braze reactions. The braze viscosity changes due to both compositional and thermal changes, thereby affecting flow velocity, terminal flow distance and flow time. For example, at higher brazing temperatures, a liquid braze can exhibit better spreading characteristics into the gap or clearance but often at the expense of greater base metal dissolution by the filler metal. The geometry of the discontinuity (for example, the gap) to be brazed also affects the flow characteristics.
In typical brazing methods using a braze furnace, the base metal hardware and the filler metal are enclosed in a heated chamber under a controlled atmosphere at a generally uniform temperature. The brazing process is generally performed with slow heating and cooling to minimize detrimental temperature gradients within the furnace and residual stresses in the brazed parts. Using such a process does not allow fine control of the filler metal flow characteristics or the braze joint dissolution and stress characteristics and can take from minutes to hours for the slow heating rates typically used, depending on the work mass.